This application claims priority of DE 198 55 404.4-21, filed Dec. 1, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a hydraulic power steering system for non-tracked vehicles, particularly motor vehicles, having a steering handle, such as a manual steering wheel, operated on the driver side and a double-acting hydraulic servo motor (steering actuator) positively coupled with steered vehicle wheels for their steering adjustment,
A motor-operated, particularly electrically operated servo valve arrangement is connected by a pressure connection with the delivery side of a hydraulic pump, by way of a low-pressure connection with the suction side of the hydraulic pump or a hydraulic reservoir communicating with the suction side and, by way of two motor connections, with the two sides of the servo motor. When the pump is operating, the control of a pressure difference between the motor connections with respect to the extent and the direction is permitted. An electronic automatic control arrangement has a desired steering angle value generator operated by the steering handle as well as an actual steering angle value generator operated by the steered vehicle wheels and controls the servo valve as a function of a desired-actual value comparison.
In hydraulic power steering systems currently used in motor vehicles, a manual steering wheel and the steered vehicle wheels are mechanically positively coupled with one another. Typically, a double-acting piston-cylinder unit is used as the servo motor. As a function of forces and moments effective between the manual steering wheel and the steered vehicle wheels, the servo motor is controlled such that only limited forces must be applied to the manual steering wheel by the driver. The positive coupling between the manual steering wheel and the steered vehicle wheels comprises two parts which can be moved relative to one another and whose relative movements with respect to the extent and direction are analogous to the forces and moments effective between the manual steering wheel and the steered vehicle wheels. These relative movements are transmitted to a servo valve controlling the servo motor. Thereby, servo forces can be generated which are analogous to the forces and moments effective between the manual steering wheel and the steered vehicle wheels.
In addition, steering systems are currently contemplated for normal road vehicles in the case of which any mechanical positive coupling between the steering handle or manual steering wheel and the steered vehicle wheels is absent. On the contrary, the steering handle or manual steering wheel and the steered vehicle wheels in the normal operation are coupled with one another by a control system only with respect to the effect, in which case the steering handle or manual steering wheel operates a desired-value generator for the respective desired steering angle which is then adjusted by the control system at the steered vehicle wheels with which an actual steering angle value generator is associated.
In this case, the control system may take additional parameters into account and can change, for example, the transmission ratio between the adjusting or rotating stroke of the steering handle or manual steering wheel and the steering angle change of the steered vehicle wheels as a function of the vehicle speed. In addition, if required, a steering against swerving motions of the vehicle and cross wind influences can automatically take place before the driver is capable of carrying out a suitable steering correction.
DE 196 27 893 C1 discloses a steering system in which, in normal operation, no positive coupling exists between the steering handle and the steered vehicle wheels. For emergency purposes, a hydraulic coupling is provided between the steering handle and the steered vehicle wheels which is normally switched to free wheeling. This hydraulic coupling is automatically switched to be operative as soon as a defect is detected in the electronically controlled main system which operates without a positive coupling between the steering handle and the steered vehicle wheels. Such steering systems thus implement the xe2x80x9csteer by wirexe2x80x9d concept.
An object of the present invention is to provide a construction which is advantageous for a xe2x80x9csteer by wirexe2x80x9d purpose.
According to the invention, this object has been achieved in that, forming two or several parallel hydraulic circuits, the servo valve arrangement as well as the pump are arranged twice or several times. A change-over valve arrangement between the hydraulic circuits and the servo motor connects in any of its positions in each case a different hydraulic circuit with the servo motor and shuts off the other hydraulic circuits with respect to the servo motor. An electronic monitoring device combined with a sensor system for detecting operating parameters of the hydraulic circuits and/or with the automatic control arrangement, in the event of disturbances in the hydraulic circuit connected with the servo motor and/or in the control system of the control arrangement assigned to this hydraulic circuit, triggers the change-over valve arrangement in order to connect an undisturbed hydraulic circuit or a hydraulic circuit with an undisturbed control system with the servo motor.
The present invention is based on the recognition of providing at least two equivalent parallel hydraulic circuits which are controlled and monitored separately from one another, of which, however, only one is effectively connected with the servo motor. In the event of a disturbance, the servo motor can then be connected with a fully operable hydraulic circuit, in which case no concern about a noticeable change of the operating behavior of the steering system. That is, the hydraulic circuit, which now communicates with the servo motor notwithstanding its separation from the servo motor, has carried out all operating steps required for controlling the servo motor. During the change-over of the change-over valve, the respective subsequent hydraulic circuit therefore takes over, without any interruption, the work of the hydraulic circuit previously connected with the servo motor.
According to a currently contemplated particularly preferred embodiment of the present invention, the servo motor and the change-over valve form a constructional unit such that no leakages can occur between the servomotor and the change-over valve. Furthermore, all parts of the hydraulic systems can advantageously be under a certain excess pressure. As a result, air can be securely prevented from penetrating into the hydraulic system.
It this context, it may be advantageous to construct each hydraulic reservoir as a low-pressure reservoir, so that a hydraulic prestressing is ensured also on the suction side of the hydraulic pumps.
If it is ensured that the pumps can take in no air, it can optionally also be provided to keep each hydraulic reservoir without pressure and to arrange a throttle in the return lines leading from the servo valves back to the reservoir. Thereby, during the operation of the pump, a minimum pressure occurs on the input side of this throttle and thus at all connections of the servo valve.